An polyolefin based thermoplastic elastomer or plastomer is a polymer composite of such as a random copolymer represented by ethylene-α-olefin copolymer, and has been widely used in industry such fields as films, sheets, fibers, non-woven fabrics, various containers or molded products, and modifiers because of it's appropriate flexibility and strength, also low environmental load in respect of recycling or burning disposal, and further satisfactory light weight, processability and economic efficiency.
Among these thermoplastic elastomers, a reactor TPO obtained through sequential polymerization that composed of a crystalline polypropylene produced in the first step and a propylene-ethylene copolymer elastomer produced in the second step (such propylene based polymer is generally referred to as “block copolymer”) has the advantages of heat resistance and productivity as compared with a random copolymer elastomer, and of the abilities to provide a product having a stable quality in the wide range of it's compositions at a low production cost as compared with an elastomer composite produced by mechanically mixing. Accordingly, this reactor TPO has high economic efficiency and excellent heat resistance and strength, and is recently widely used.
However, many of them have defects that their transparency is remarkably poor and their flexibility is not enough, because a crystalline polypropylene produced in the first step and a propylene-ethylene copolymer elastomer produced in the second step are immiscible and show a phase-separated structure.
Under these circumstances, a polyolefin based elastomer or plastomer having excellent transparency and flexibility is advantageous for industrial uses, and therefore various improvements have been made heretofore.
For example, in order to improve flexibility and transparency, there is a proposal of sequential polymerization by a Ziegler-Natta catalyst, which comprises a pair of steps, in the first step, a polypropylene or propylene-ethylene copolymer containing small amount of ethylene is prepared, and in the second step, a propylene-ethylene copolymer elastomer containing relatively low but higher than that of the polymer component obtained in the first step (see Patent Publication Documents 1 and 2). However, since a Ziegler-Natta catalyst has various kinds of active sites, the propylene-ethylene random copolymer thus produced has broad composition distribution and broad molecular weight distribution. And such a polymer contains a considerable amount of low crystallinity and/or low molecular weight components. Accordingly, such products obtained thereof are sticky and poor in bleed-out (leaching of low molecular weight component or additives), and have defects of easily causing problems such as blocking or poor appearance.
In order to solve this problem, a modification of the former method is proposed that an intrinsic viscosity (i.e. a molecular weight) of an elastomer is increased higher than a certain level to suppress the formation of a low molecular weight component (see Patent Publication Document 3). However, even when the molecular weight is increased, the formation of a low crystalline component is not efficiently suppressed and transparency is not sufficient and stickiness or bleed-out problems are not sufficiently improved. Further, the higher molecular weight elastomer causes a poor appearance due to the presence of grains (generally called fisheyes (FE) or the like), and a poor processability, which need an addition of an organic peroxide for rheology control.
Thereafter, a reactor TPO obtained by a metallocene catalyst has been developed. For example, a propylene based copolymer having a specific elution pattern in temperature rising elution fractionation (TREF) was obtained by the sequential polymerization that a polypropylene was prepared in the first step and a copolymer of propylene and ethylene and/or a C4-C18 α-olefin was prepared in the second step. This copolymer showed no tackiness as an improved feature because of narrow composition distributions and narrow molecular weight distribution (see Patent Publication Documents 4 and 5).
Particularly, in order to solve the above-mentioned problems, Patent Publication Document 4 disclosed a propylene based polymer composition comprising a polypropylene component and a propylene-ethylene copolymer component, being satisfied with requirements that a component eluted under 90° C. is from 50 to 99 wt % to the total components, a component eluted above 90° C. is from 50 to 1 wt %, and a component eluted under 0° C. is at most 10 wt % in a temperature rising elution fractionation using o-dichlorobenzene solvent.
However, the propylene based polymer composition obtained by this method had a high melting temperature and was not suitable for processing at low temperature, particularly calendaring, and also had a problem that heat resistance remarkably declined in the case of the specific composition for highly improved flexibility.
Also, Patent Publication Document 5 discloses a composition containing large amount of the eluted fraction under 0° C. in TREF by increasing an ethylene content in the second step.
This composition had excellent flexibility but poor heat resistance. And this composition took a phase-separated structure, so there were various problems of a poor transparency evaluated by a total light transmittance although a haze value is not high, and of a poor appearance due to flow mark.
By the way, a polyolefin film is excellent in physical properties, transparency, processability, economical efficiency and low environmental load, and is accordingly widely used as a packaging material in various fields mainly for foods, daily necessaries, pharmaceuticals, miscellaneous goods and the like.
Further, various functions are required in packaging material, so many kinds of films are used, such as an oriented film being excellent in strength and stiffness, a heat shrinkable film for adhesive packaging, a gas barrier film for storing, a retort film for boiling sterilization and so on.
Among these various functions, one of the most important function is a heat sealing property for a polyolefin film as a packaging material.
Mechanical properties such as strength and stiffness of a polyolefin film are enhanced by orientation.
But, a crystalline polypropylene oriented film has poor heat sealing properties. For example, heat sealable temperature is high, heat sealable temperature range is narrow, heat sealing strength is not enough, a slippery property and an anti-blocking property are poor, and packaging operatability is not satisfactory.
To solve these problems, a polypropylene based oriented multi-layer film having a surface layer with good heat sealing properties is proposed.
But a conventional heat sealing material such as low density polyethylene is poor in transparency and anti-blocking property, and a propylene-ethylene random copolymer containing very low amount of ethylene is poor in low temperature heat sealing property although transparency and anti-blocking property are satisfactory.
When an ethylene content is increased in propylene-ethylene random copolymer, its low temperature heat sealing property is improved, but anti-blocking property becomes poor. Even by using a copolymer of propylene and butane-1, slippery property, anti-blocking property, and packaging operatability is not satisfactory, or heat sealing strength is not satisfactory although low temperature heat sealing property is improved (see Patent Publication Document 6).
A multi-layer film having a surface layer using a composition of propylene-ethylene random block copolymer and polyolefin is improved in heat sealing property, slippery property and tearing property (see Patent Publication Document 7), a co-extruded multi-layer film improved in heat sealing property and slippery property by using a composition of propylene-ethylene random block copolymer having different ethylene contents as a surface layer (see Patent Publication Document 8), a co-extruded multi-layer film improved in heat sealing property by using a propylene-ethylene block copolymer having an elution component specified by temperature rising elution fractionation method (see Patent Publication Document 9), a multi-layer film improved in heat sealing property and impact strength by using a propylene-α-olefin block copolymer as a surface layer (see Patent Publication Document 10), a multi-layer film improved in heat sealing property, anti-blocking property and stiffness by using a composition of propylene based polymer and propylene-α-olefin random copolymer as a surface layer (see Patent Publication Document 11), and a co-extruded multi-layer film improved in heat sealing strength by improving thickness accuracy of a biaxially oriented base layer by using a composition of propylene homopolymer and propylene-ethylene random copolymer as a base layer and a propylene-α-olefin copolymer as a surface layer (see Patent Publication Document 12).
However, even these improving techniques do not satisfy following all of the properties at the same time, i.e., heat sealing strength, low temperature heat sealing property, transparency, slippery property and anti-blocking property.
Patent Publication Document 1: JP-A-63-159412 (Claims, page 2, right lower column)
Patent Publication Document 2: JP-A-63-168414 (Claims, page 2, right lower column)
Patent Publication Document 3: Japanese Patent No. 3358441 (Claim 1)
Patent Publication Document 4: JP-A-2000-239462 (Abstract)
Patent Publication Document 5: JP-A-2001-64335 (Abstract)
Patent Publication Document 6: JP-A-8-238729 (Claim 1, paragraphs 0001, 0003 and 0005 to 0013)
Patent Publication Document 7: JP-A-9-227757 (Abstract, Claim 1)
Patent Publication Document 8: JP-A-9-239925 (Abstract)
Patent Publication Document 9: JP-A-2000-239462 (Abstract, Claim 1, Claim 6 and Claim 7)
Patent Publication Document 10: JP-A-2001-59028 (Abstract, Claim 4)
Patent Publication Document 11: JP-A-2003-55510 (Abstract)
Patent Publication Document 12: JP-A-2003-170555 (Abstract, Claim 1, paragraph 0004)
As described above, the main object of the present invention is to develop an industrially useful polyolefin elastomer or plastomer material by improving transparency, flexibility and heat resistance.
Further, the present invention is directed to a propylene-ethylene random block copolymer useful as a material excellent in processability, which suppresses stickiness and bleed-out so as not to cause a poor appearance due to FE.
Still further, in view of the prior art discussed above, another object of the present invention is to propose a polyolefin based biaxially oriented multi-layer film which has excellent performance in following all the properties, heat sealing strength, low temperature heat sealing property, transparency, anti-blocking property and slippery property.
In order to obtain a novel random block copolymer, the present inventors have discovered that important requirements are to select a kind of catalyst and a specific polymerization method, to specify crystallinities both of a high crystalline component and a low crystalline or an amorphous component in the copolymer and to specify their composition.
In detail, the present inventors have discovered that important factors for preparing the above-mentioned novel and excellent propylene-ethylene random block copolymer are to employ a metallocene catalyst, to employ sequential (multi-step) polymerization of a block copolymer as a polymerization method, to be composed of a specific crystalline propylene-ethylene random copolymer component having a narrow crystallinity distribution and a specific low crystalline or a specific amorphous propylene-ethylene random copolymer component in a specific amount.
Furthermore, in the present invention the extremely flexible and transparent random block copolymer can be obtained keeping an adequate heat resistance, by increasing ethylene content in crystalline component in the first step instead of increasing an amount of low crystalline or amorphous component in the second step too much.
On the other hand, the present inventors apply the specified propylene-ethylene random block copolymer as a surface layer of the film to improve above-mentioned properties, i.e., heat sealing strength, low temperature heat sealing property, transparency, appearance, anti-blocking property and slippery property. In order to improve these properties, it is recognized that these properties are closely related to polymerization method, composition of respective components, comonomer content of each component, molecular weight, molecular weight distribution, miscibility of respective components, and crystallinities. The present invention has been conceived on the basis of this discovery.
For example, in order to obtain a high heat sealing strength, it is important that the surface layer has an appropriate flexibility, which mainly depends on a composition of respective components and an comonomer content of the random block copolymer. On the other hand, in order to improve transparency, low temperature heat sealing property and anti-blocking property, it is an important that the random block copolymer is made by a specific polymerization method with a specific kind of catalyst in the preparation and that obtained random block copolymer has a specific crystallinity and a specific composition. Precisely speaking, the transparency depends on miscibility of the random block copolymer, in other words, depends on the crystallinity (comonomer content) and molecular weight of two components of the random block copolymer. The heat sealing property and anti-blocking property depend on a crystallinity, molecular weight, molecular weight distribution, a composition of respective components and a comonomer content.
On the basis of the above-mentioned consideration and experimental analyses, the present invention basically employs the propylene-ethylene random block copolymer having specific characteristics obtained by sequential polymerization employing a metallocene catalyst as a surface layer of the polyolefin based biaxially oriented multi-layered film. The random block copolymer is obtained through sequential polymerization which is composed of 30 to 70 wt % of a propylene-ethylene random copolymer component having an ethylene content of 1 to 7 wt % in the first step and 70 to 30 wt % of a low crystalline or an amorphous propylene-ethylene random copolymer component in the second step having an ethylene content of 6 to 15 wt % higher than that of the polymer component obtained in the first step. Further, in order to specify miscibility which is deeply related to transparency, temperature-loss tangent (tan δ) curve obtained by Dynamic Mechanical Analysis (DMA) is employed. When this tan δ curve shows a single peak at 0° C. or lower, the propylene-ethylene random block copolymer is miscible and a film obtained therefrom is highly transparent.
JP-A-8-244182 discloses a multi-layer film excellent in peeling property and in tear property, which has surface layer comprising a propylene based block copolymer having Mw/Mn range of 1 to 4 and being composed of two component 1 to 70 wt % of component(A) and 30 to 99 wt % of component(B), where component(A) is polypropylene or propylene based random copolymer having at least 90 mol % propylene content and component(B) is propylene-ethylene random copolymer having 10 to 40 mol % ethylene content.
However, this technique is directed to improve peeling property and tear property only, and to obtain high heat sealing strength, it is necessary to employ significantly large amount of a propylene-ethylene random copolymer component(B). In such a case, there is a problem that it is necessary to employ a relatively large amount of additives for keeping anti-blocking property. Further, to obtain sufficient heat sealing strength, there is a problem that it is necessary to increase a thickness of a surface layer extremely, however this method is generally undesirable from an economical point of view.
The present invention partly overlaps with this prior art with respect to a composition of components in a propylene-ethylene random block copolymer, however the present invention focuses on improving not peeling property and tear property but heat sealing strength, low temperature heat sealing property, transparency, anti-blocking property and slippery property. To achieve these purposes, this invention specifies not only the composition and comonomer contents of the random block copolymer but also a kind of a catalyst and a polymerization method. Moreover, this invention specifies loss tangent properties closely related mainly to transparency. Thus, it is clear that the present invention is substantially different from this prior art technique.